1. Field of the Invention
The present invention concerns a method for reducing and more preferably for eliminating hot sticking between a die, particularly an electrically conducting die, and an inorganic non-metallic material (=an insulator), particularly an insulator of which at least a part is in vitreous state, being in contact with each other at a temperature where the insulator is moldable. The invention is furthermore directed to a process for producing a molded insulator with improved surface quality by using said method and to a device to perform said method.
2. Description of Related Art
Numerous undesirable hot sticking phenomena occurring in complex industrial processes can be attributed to a general problem of hot sticking between a forming die which in general is made from a metal or another electrically conducting material and an inorganic non-metallic material (=insulator), in particular an inorganic insulator like glass, enamel and ceramic, of which at least one of the phases which constitute it is viscous at the temperature in question.
During the hot molding of inorganic insulating materials like glass or enamel or combinations therefrom at a temperature for which said materials have a viscosity of between 1 and 10,000 kPa.multidot.s (=10.sup.4 and 10.sup.8 poises) sticking phenomena are encountered between said insulating materials and the forming die or the mold. The forming die may be entirely metal, or ceramic covered with a metallic coating, or metal or ceramic covered with a ceramic coating sufficiently electrically conductive at the molding temperature. The temperature range corresponding to said viscosities is variable according to the composition, from 500 to 1000.degree. C. for certain industrial silica-alkali-lime glasses. These phenomena have appeared in the course of the development of new technologies. For example:
1. Sticking of a molten glass to the metal mold during the manufacture of flasks, bottles, beakers. PA1 2. Sticking of an enamel (mixture of a glass in powder form which is called frit and of a crystallized coloring agent which is called pigment) to the metallic forming die during the forming of glazings of automobiles.
This list is not at all exhaustive.
Some solution have been found for enamels for glazings of automobiles:
Addition of zinc and tin oxides, or of copper sulphates, which act as "anti-stick" components, that is to say they are components which contribute the property of non-sticking--see U.S. Pat. Nos. 4,684,388 (Ford Motor Company) and 4,828,596 (Ciba-Geigy Corporation).
Development of glass frits which recrystallize on stoving--see EP 0 370 683 A1 (Johnson Matthey Public Limited Company) and U.S. Pat. No. 5,153,150 (Ferro Corporation).
This type of solution is only partial however. The success of said compositions depends to a large extent on the forming conditions, and more particularly on the stoving cycle of the enamels.
It is an object of the present invention to limit the sticking problems without acting on the compositions. It is a further object to provide a process for any kind of molding of at least partly vitreous inorganic materials which leads to a better surface quality of the molded article by eliminating or at least reducing said hot sticking problems. Still another object is directed to a device comprising the usual equipment which permits said molding and means by which said sticking phenomena can be reduced or essentially be avoided.